Note: Descriptions are shown in the official language in which they were submitted.
91-rTRI~-472
~ ~8 33~
A SEMICONDUCTOR DEVICE COOLING SYSTEM USING A HEAT SINK
ArrACHED TO A CIRCUIT BOARD CONTAINING HEAT CONDUCTIVE
LAYERS AND CHANNELS
Related ~pplications
The present application relates to Canadian Application 2 097 097
entitled "Heat Sink Mounting for Semiconductor Devices~ filed on May 27 1993
and assigned to the same assignee Eaton Corporation as this application.
Field of the Invention
This invention relates to a method of mounting a power eleclrical
device to a printed circuit board having a heat diss~r~tion device attached to
10 the opposite side of the board. More srecifice~y, the presel)l invention relates
to a method of mounting a power electrical device to a circuit board where the
heat ge"eraled by the device is con~cte~l into a heat sink located on the
oprosite side of the ~.rinlecl circuit board through a plurality of heat condlJction
chal "~els and further con~llJcte~ and tJiss;p~ed by a plurality of heat conducting
15 layers within the printed circuit board.
Background ot the Invention
It is com,non to use a heat ~J:ssil~;o" device one type being
commonly known as a heat sink which is placed in contact with the back
20 surface of an electrical device such as a semiconductor for effectuating a
cooling function to prevent an over temperal.lre condition of the device when
operating at high power levels. The device is attached to the heat sink usually
using a multiplicity of ~aslenefs which require very accurate ",acl,ining and a
high degree of assembly labor especially when a large number of devices are
25 sequentially mounted.
Heat conduction is improved with the use of a solder or ~ heat
conductive paste applied to the mounting side of a thermal pad which is a part
of the semiconductor device and is usually boltea to a heat sink through a hole
in the flange of the thermal pad. The thermal pad is attached to the device
30 active elements and functions as a heat conductive plate whose purpose is to
A
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conduct the heat generated by the elements and is directly attached to a larger
more effective heat ~issip~tor such as a heat sink which requires a large
amount of space on the printed circuit board.
It is also known to use a cooling jacket to cool the devices where
the devices themselves are immersed in a cooling fluid or the devices are
attached and in thermal contact with a heat sink which contains one or more
cooling channels wherein a cooling fluid is forcibly circulated.
rl oblei "s have arisen when the electrical device such as a power
semiconductor is mounted to a circuit board for cooling by heat llansfer,ed
through the heat conductive thermal pad and where electrical con"e~io" leads
that emanate from the device are attached to connection pads known as
solder pads on the printed circuit board. To obtain adequate cooling an
auxiliary heat sink must be added in some manner to effectively conduct
additional heat from the thermal pad of the ele t,ical device to control the
operating temperature. It is known to mount the semiconductor directly to a
heat sink having fins which extend into the surrounding air and are mounted to
the printed circuit board however, this a~Jp,oach is not conducive to high
packaging density on the printed circuit board itself since the heat sink requires
a large area which results in a larger overall size of the electronics module.---lt
is thus desirable to provide a method of mounting the heat sink such that it caneffectively conduct heat away from the semiconductor device while allowing for
dense packaging of the electrical co" ,pone, lls on the printed circuit board with
a minimum number of fasteners and a relaxalion of the geo",etric tolerances
involved.
The stringent yeo",el,ic requirel"enls and accuracy of the prior
art methods and high labor content are not conducive to high volume
manufacturing asse",bly of the printed circuit board where the heat sin!~ is
mounted directly to the se" ,iconductor device i"c, easiny cost especially wherea multiplicity of devices are mounted sequentially. Also the prior art metl,o;Jsdo not generate suf~icient clamping loads on the electrical device to establish
a good thermal conductivity between the heat conductive thermal pad and the
heat sink. It is desirable to have a large contact area between the
3 3 ~
semiconductor thermal pad and the heat sink to maximize the heat Ira.1srer and
the cooling effect. Prior art methods, such as that ~Jesc,ibèd in U. S. ~atent
No. 4 479 140 U. S. Patent No. 5 089 936 and the aforementioned Canadian
Application 2 097 097 describe methods to use springs to assure contact
5 between a semiconductor device and the heat sink.
These methods are effective but require additional parts, such as
10 sprinys mounting bolts and a retenlion block. Prior art devices do not allow
for the semiconductor device to be mounted directly on the circuit board so thatthe electrical leads can be soldered to the printed circuit board soldering padsto minimize induced stresses either at the solder pads or along the electrical
leads themselves while allowing for high cla",ping Ioads to maximize the heat
15 conduction and into the heat sink. Also the heat sink is mounted on the same
side of the circuit board as the semiconductor device which does not allow for
optimum location of the electrical components for dense packaging and
compl cates the assembly ~,rocess.
20 Summary ot the Invention
The prese.,l invention provides a method of mounting a power
electrical device such as a semiconductor directly to a printed circuit board bysoldering the heat condlJction thermal pad of the semiconductor device to the
surface of the board where the electricai leads ema,1ali-)9 ll,eret,o", are
25 soldered to individual solder pads which are electrically connected to the other
electrical components located on the board. The heat generated by the
semiconductor device when in operation is conducted through a plurality of
heat conductive layers one located on the top surface of the board and to
which the semiconductor thermal pad is mounted and one on the bottom
30 surface with the balance evenly spaced therebetween and embedded in the
board. A plurality of heat conduction channeis transverse to the layers are
used to conduct the heat from layer to layer and finally to the heat conduction
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layer that is located on the bottom side of the printed circuit board. The heat
that is conducted through the heat con~uction c~,ai,r,els and into the heat
conduction layer located on the bottom of the circuit board is then l~nsterred
through a compliant electrically insulating layer which has a relatively high
5 thermal coefficient of conductivity into a heat sink which is co,ni"only made out
of an aluminum block which can include fins to maximize the radiated heat
transferred into the surrounding air. In this " ,a",)er, the semiconductor device
can be simply mounted on the top surface of a printed circuit board without the
use of springs and fasteners while allowing the electrical leads to be soldered
10 to solder pads on the circuit board thereby minimizing mechanical stresses
when high temperatures are encountered. The circuit board is clamped to the
heat sink with relatively large mounting bolts positioned between the
semiconductor devices and aligned with the center axis of the heat conduction
channels to produce high clamping forces to maximize heat transfer.
A provision of the present invention is to provide for a method of
conducting the heat generated from an elect,ical device through a plurality of
heat conduction layers and heat conducting channels into a heat sink that is
mounted on the opposite side of a printed circuit board from the other
~ components.
Another provision of the present invention is to maximize the
component density of a printed circuit board by allowing the heat dissipative
members, such as a heat sink, to be mounted on a botlGm side of a prinled
circuit board where the electrical components are mounted on top side of the
pri"led circuit board.
Another provision of the present invention is to allow for relatively
high cla~ .)g loads to be introduced between the heat sink and the circuit
board to provide for a high level of heat conductivity between a heat conductinglayer and the heat sink.
Still anoLl)er provision of the ,ureser)t invention is to allow for a
30 method of cooling an ele~;tlical device, which is mounted to a circuit board and
in turn mounted to a heat sink, using a minimum of me~:l,anical rasleners for
ease of assembly.
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Brief Description of the Drawings
Figure 1 is a sectional view of the present invention showing a
semiconductor device mounted to a circuit board which is securely clamped to
a heat sink; and
Figure 2 is a top view of Figure 1 showing the method of mounting
a semiconductor device of the present invention.
Detailed Description of the ~lef~r,~ Embodiments
In accortJance with the present invention, as shown most clearly
in Figures 1 and 2, at least one electrical device such as semiconductor device
4 having an integral heat conduction pad 5 is mounted to a printed circuit board6 where the assembly thereof is secured to a heat ~issir~ting device, such as
an aluminum heat sink 8, which functions to dissirate heat generated by the
semiconductor device 4. The generated heat passes through the prinle~ circuit
board 6 into the heat sink 8 which is then dissirated into the surrounding air
usually by means of a plurality of fins formed in the heat sink. Other methods
can be used to conduct the heat, such as a forced circulation of liquid through
the heat dissip~tion device. All of these heat dissipation devices function to
maintain a proper operating temperature of the semiconductor device 4 so that
temperature extremes are not encountered which can degrade the performance
of the semiconductor device 4 or cause failure either immediately or over a
period of time.
The printed circuit board 6 contains a plurality of heat conducting
layers 10 which can be made of a material having a high value of
thermoconductivity such as copper. The surface of the heat conduction layers
are a,u~.ro~i,,,alely parallel to the top and bottom surfaces of the prir,led circuit
board 6. Figure 1 shows a printed circuit board 6 having a copper layer 10 on
a top surface and a L~utloi 1 I surface of the printed circuit board 6 and also two
layers evenly s~ aced therebetween and embedded within the printed circuit
board 6 wh~ch function to conduct heat and distribute heat one to the other
through a plurality of heat conduction channels 12 whose axis lie approximately
perpendicular to the surface of the heat corducting layers 10. The heat
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conduction cl ,a, l~ lels 12 are preferably formed by drilling or punching a plurality
of holes into and through the printed circuit board 6 and then filling those holes
with a heat conductive material such as solder. The heat condlJction channels
12 function to transfer heat by con~uction from one heat conducting layer 10
5 to another and finally into the heat sink 8 for dissipation into the al",os~-l,ere.
Figure 2 more clearly shows the ~lia")eter of the heat cond~ ~ction
channels 12 which are maximized in area to provide for a more effective heat
conduction from the semiconductor device 4 into the heat condlJction thermal
pad 5 which is attached to the ~rinled circuit board 6 using semiconductor
10 mounting solder 20 and finally into a first heat conducting layer 10 where the
heat conduction channels 12 transfer the heat to each conducting layer 10 and
finally through a heat conductive electrical insulator 19 and into the heat sink8.
The printed circuit board 6 is held firmly to the heat sink 8 through
15 the heat conductive electrical insulator 19 clamped between the printed circuit
board 6 and the heat sink 8 by a ,necl,anical clai"ping fastener 16 which can
be a mechanical screw which engages a threaded opening 18 locate,l in the
heat sink 8. The mechanical clamping faslener 16 also engages a cla,npi"g
plate 14 passing through a hole therein where the cla,n~.ing plate 14 improves
20 the distribution of the clarn,~ing load generaled by the mechanical clamping
fdsle,ler 16. Very high cla" ",i"g loads can be induced by the clamping fastener16 and are desirable to provide for effective heat l,~"sfer.
The function of the heat conductive ele-1,ical ins~ or 19 is to
electrically insulate the printed circuit board 6 from the heat sink 8 where these
25 two elements are collllllurlly at diff~rellt levels of electrical potel,lial and must
be electrically insulated while allowing for heat lransfer from the ~.ri"led circuit
board 6 into the heat sink 8 for thermal control.
The semiconductor device 4 often contains a plurality of ele~rical
leads 22 which extend and engage holes in the prinlecl circuit board 6 and are
30 soldered to solder pads 24 which are in turn conne.:ted to a plurality of other
electrical devices located on the printed circuit board 6.
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Using the present invention, a series of semiconductor devices 4
can be conveniently mounted onto the top surface of a ~.rinted circuit board 6
where the heat sink 8 is mounted to the bottom surface of the printed circuit
board Ihereby allowing for a very dense packaging thereby minimizing the size
5 of the printed circuit board 6.
Where a multiplicity of semiconductor devices 4 are mounted, a
cla" ~ ,9 tasle"er 16 is prererably located between each pair of semiconductor
devices 4 which provide in combination with the clamping plate 14 for high
clamping loads thereby maximizing the heat l,d"srer from the printed circuit
10 board 6 and the heat conducting layers 10 into the heat sink 8. The clamping
faslener~ 16 are approximately aligned with the central axis of the section of the
heat conductive layer containing the heat conductive channels.
It will be appreciated by those of orcJina, y skill in the art that many
variations in the foregoing preferl ed embodi" lenls are possible while remaining
15 within the scope of the present invention. The present invention should thus
not be considered limited in the preferred embodiments where the specific
choices of materials, configulaliGns, dimensi~ns, arplirations or ranges of
parameters employed therein.
